Field of the Invention
The present invention relates to a rotation speed measuring system for measuring the rotation speed of a ball moving in midair.
Description of the Related Art
As a rotation speed measuring system for measuring the rotation speed of a ball moving in midair, a rotation speed measuring system is known that includes a magnetic sensor that measures earth magnetism (see Patent Literature 1 described below). In the rotation speed measuring system, the magnetic sensor is provided in a ball, and temporal changes in output of the magnetic sensor are used to calculate the rotation speed of the ball moving in midair.
When a ball is thrown or kicked, the ball moves in midair while rotating. Rotation of the ball moving in midair causes a cyclic change in direction of the magnetic sensor provided in the ball with respect to earth magnetism. Thus, measurement data on the earth magnetism acquired by the magnetic sensor shows a cyclic change. The above rotation speed measuring system is configured to analyze the frequency of the cyclic change, so as to calculate the rotation speed of the ball moving in midair.